1. Field of the Invention
The present invention relates to novel precipitated silica particulates, characteristically in the shape of granules, powders or substantially spherical beads, to a process for the preparation thereof, and to the use of same as a reinforcing filler material for elastomeric/rubbery matrices.
2. Description of the Prior Art
Precipitated silica particulates have long been used as a white reinforcing filler in elastomers and, particularly, in tires for the automotive industry.
As is the case with any reinforcing filler material, however, it must be (a) easy to handle and (b) easy to formulate into mixtures thereof.
In this respect, the powder form is not always satisfactory in that, purely from the standpoint of handling and formulation, it may promote considerable dust formation and tedious and inadequate incorporation of the filler (low apparent density). Furthermore, rubber mixing requires very accurate metering techniques, for which powdery fillers are often unsuitable (pourability).
Shaping such filler material into granule form is of course one suitable technique for obviating the above disadvantages, but, unfortunately, it may often provide insufficient dispersion of the filler in the elastomer, and the degree of reinforcement ultimately attained may be lower than could be obtained using a filler initially in powder form.
In general, it is known to this art that, if a filler is to provide optimum reinforcing properties, it must be present in the elastomer matrix in a final form which is both as finely divided as possible and as homogeneously distributed as possible. Thus, in the special case of the filler being introduced initially in granular state, such conditions can be realized only insofar as (a) the granules have a very good capacity for incorporation into the matrix when mixed with the elastomer (incorporability of the granules) and for disintegration or deagglomeration in the form of a very fine powder (disintegration of the granules), and (b) the powder resulting from such disintegrating process can in turn be dispersed completely and homogeneously within the elastomer matrix (dispersion of the powder). It will readily be appreciated that these requirements are incompatible, or even conflict with the very nature of a granulate, considering its inherently dense, compact and resistant character and the relatively high cohesive strength bonding the constituent grains of silica. Preparation of granules of precipitated silica without sacrificing either mechanical strength or capacity for dispersion to date remains a problem of compromise which is difficult to overcome.
Another difficulty resides in that fact that, for reasons of mutual affinity, the silica particles (whether or not they emanate from a prior disintegration of granules) have an unfortunate tendency to agglomerate within the elastomer matrix. These silica/silica interactions promote the deleterious effect of limiting the reinforcing properties to a level well below that which could theoretically be attained if all of the silica/elastomer interactions possible during the mixing operation were in fact obtained. (The theoretical number of silica/elastomer interactions is known to be directly proportional to the external surface area or CTAB surface area of the silica employed).
Moreover, such silica/silica interactions tend to increase the stiffness and thickness of the mixtures, thus rendering them more difficult to use.